1. Field of the Invention
The present invention pertains to wireless telecommunications, and particularly to signaling required for supporting common transport channels in a radio access network for a radio connection with a user equipment unit controlled by a serving radio network controller (SRNC) when the user equipment unit moves into a new cell controlled by a drift radio network controller (DRNC).
2. Related Art and Other Considerations
In a typical cellular radio system, mobile user equipment units (UEs) communicate via a radio access network (RAN) to one or more core networks. The user equipment units (UEs) can be mobile stations such as mobile telephones (“cellular” telephones) and laptops with mobile termination, and thus can be, for example, portable, pocket, hand-held, computer-included, or car-mounted mobile devices which communicate voice and/or data with radio access network.
The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station. A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by a unique identity, which is broadcast in the cell. The base stations communicate over the air interface (e.g., radio frequencies) with the user equipment units (UE) within range of the base stations. In the radio access network, several base stations are typically connected (e.g., by landlines or microwave) to a radio network controller (RNC). The radio network controller, also sometimes termed a base station controller (BSC), supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
One example of a radio access network is the Universal Mobile Telecommunications (UMTS) Terrestrial Radio Access Network (UTRAN). The UTRAN is a third generation system which in some respects builds upon the radio access technology known as Global System for Mobile communications (GSM) developed in Europe. UTRAN is essentially a wideband code division multiple access (W-CDMA) system.
As those skilled in the art appreciate, in W-CDMA technology a common frequency band allows simultaneous communication between a user equipment unit (UE) and plural base stations. Signals occupying the common frequency band are discriminated at the receiving station through spread spectrum CDMA waveform properties based on the use of a high speed, pseudo-noise (PN) code. These high speed PN codes are used to modulate signals transmitted from the base stations and the user equipment units (UEs). Transmitter stations using different PN codes (or a PN code offset in time) produce signals that can be separately demodulated at a receiving station. The high speed PN modulation also allows the receiving station to advantageously generate a received signal from a single transmitting station by combining several distinct propagation paths of the transmitted signal. In CDMA, therefore, a user equipment unit (UE) need not switch frequency when handoff of a connection is made from one cell to another. As a result, a destination cell can support a connection to a user equipment unit (UE) at the same time the origination cell continues to service the connection. Since the user equipment unit (UE) is always communicating through at least one cell during handover, there is no disruption to the call. Hence, the term “soft handover.” In contrast to hard handover, soft handover is a “make-before-break” switching operation.
The Universal Mobile Telecommunications (UMTS) Terrestrial Radio Access Network (UTRAN) accommodates both circuit switched and packet switched connections. In this regard, in UTRAN the circuit switched connections involve a radio network controller (RNC) communicating with a mobile switching center (MSC), which in turn is connected to a connection-oriented, external core network, which may be (for example) the Public Switched Telephone Network (PSTN) and/or the Integrated Services Digital Network (ISDN). On the other hand, in UTRAN the packet switched connections involve the radio network controller communicating with a Serving GPRS Support Node (SGSN) which in turn is connected through a backbone network and a Gateway GPRS support node (GGSN) to packet-switched networks (e.g., the Internet, X.25 external networks)
There are several interfaces of interest in the UTRAN. The interface between the radio network controllers (RNCs) and the core network(s) is termed the “Iu” interface. The interface between a radio network controller (RNC) and its base stations (BSs) is termed the “Iub” interface. The interface between the user equipment unit (UE) and the base stations is known as the “air interface” or the “radio interface” or “Uu interface”. In some instances, a radio connection involves both a Serving or Source RNC (SRNC) and a target or drift RNC (DRNC), with the SRNC controlling the radio connection but with one or more radio links of the radio connection being handling by the DRNC. An Inter-RNC transport link can be utilized for the transport of control and data signals between Source RNC and a Drift or Target RNC, and can be either a direct link or a logical link as described, for example, in International Application Number PCT/US94/12419 (International Publication Number WO 95/15665). An interface between radio network controllers (e.g., between a Serving RNC [SRNC] and a Drift RNC [DRNC]) is termed the “Iur” interface.
The radio network controller (RNC) controls the UTRAN. In fulfilling its control role, the RNC manages resources of the UTRAN. Such resources managed by the RNC include (among others) the downlink (DL) power transmitted by the base stations; the uplink (UL) interference perceived by the base stations; and the hardware situated at the base stations.
Those skilled in the art appreciate that, with respect to a certain RAN-UE connection, an RNC can either have the role of a serving RNC (SRNC) or the role of a drift RNC (DRNC). If an RNC is a serving RNC (SRNC), the RNC is in charge of the radio connection with the user equipment unit (UE), e.g., it has full control of the radio connection within the radio access network (RAN). A serving RNC (SRNC) is connected to the core network. On the other hand, if an RNC is a drift RNC (DRNC), its supports the serving RNC (SRNC) by supplying radio resources (within the cells controlled by the drift RNC (DRNC)) needed for a radio connection with the user equipment unit (UE). A system which includes the drift radio network controller (DRNC) and the base stations controlled over the Iub Interface by the drift radio network controller (DRNC) is herein referenced as a DRNC subsystem or DRNS.
When a radio connection between the radio access network (RAN) and user equipment unit (UE) is being established, the radio access network (RAN) decides which RNC is to be the serving RNC (SRNC) and, if needed, which RNC is to be a drift RNC (DRNC). Normally, the RNC that controls the cell where the user equipment unit (UE) is located when the radio connection is first established is initially selected as the serving RNC (SRNC). As the user equipment unit (UE) moves, the radio connection is maintained even though the user equipment unit (UE) may move into a new cell, possibly even a new cell controlled by another RNC. That other RNC becomes a drift RNCs (DRNC) for RAN-UE connection. An RNC is said to be the Controlling RNC (CRNC) for the base stations connected to it by an Iub interface. This CRNC role is not UE specific. The CRNC is, among other things, responsible for handling radio resource management for the cells in the base stations connected to it by the Iub interface.
In the Universal Mobile Telecommunications (UMTS) Terrestrial Radio Access Network (UTRAN), on the radio interface there are Common Transport Channels and Dedicated Transport Channels. The Common Transport Channels are the uplink Random Access Channel (RACH), the downlink Forward Access Channel (FACH), uplink Common Packet Channel (CPCH), Uplink Shared Channel (USCH), and Downlink Shared Channel (DSCH). The Dedicated Transport Channels are the Dedicated Channel (DCH). The transport channels in the UMTS are described in one or more of the following specifications (all of which are incorporated herein by reference in their entirety): (1) Third Generation Partnership Project (3GPP) Technical Specification 25.211, v.3.5.0 “Physical Channels and Mapping of Transport Channels Onto Physical Channels (FDD)”; (2) Third Generation Partnership Project (3GPP) Technical Specification 25.221, v.3.5.0 “Physical Channels and Mapping of Transport Channels Onto Physical Channels (TDD)”; (3) Third Generation Partnership Project (3GPP) Technical Specification 25.331, v.3.5.0 “RRC Protocol Specification.”
The downlink Forward Access Channel (FACH) is a common transport channel used for transporting data to many different user equipment units (UEs). A multiplexing over the downlink Forward Access Channel (FACH) is achieved by including a UE identity in every FACH data transmission. The CRNC [currently assuming the role as drift radio network controller (DRNC) for a user equipment unit (UE)] schedules the data received from the serving radio network controller (SRNC) for the different user equipment units (UEs) on the downlink Forward Access Channel (FACH).
A downlink Forward Access Channel (FACH) is mapped onto a physical channel known as the Secondary Common Control Channel (S-CCPCH). Each Secondary Common Control Channel (S-CCPCH) may carry more than one downlink Forward Access Channel (FACH). Details of mapping of transport channels onto physical channels are provided in the first two of the above-listed 3GPP specifications.
In addition to transporting information related to several user equipment units (UEs) on the downlink Forward Access Channel (FACH), a further multiplexing level is used by transporting multiple logical channels for one user equipment unit (UE) over the downlink Forward Access Channel (FACH). There are three types of such logical channels. The first type is the Common Control Channel (CCCH) which extends between a CRNC and the user equipment unit (UE). A second type is the Dedicated Control Channel (DCCH) which extends between the serving radio network controller (SRNC) and the user equipment unit (UE). A third type is the Dedicated Transport Channel (DTCH) which extends between the serving radio network controller (SRNC) and the user equipment unit (UE).
The Common Control Channel (CCCH) extends between a CRNC and the user equipment unit (UE). As such, there is no direct transport of the Common Control Channel (CCCH) over the Iur interface. However, the CRNC may decide to relay certain information received on the Common Control Channel (CCCH) to the serving radio network controller (SRNC). The serving radio network controller (SRNC) may also ask the CRNC to transmit certain information over the Common Control Channel (CCCH). The transport of this information over the Iur interface is handled by the Radio Network Subsystem Application Protocol (RNSAP).
The data received/sent on the Dedicated Transport Channel (DTCH) and Dedicated Control Channel (DCCH) logical channels is transported on so-called “transport bearers” over the Iur interface. One transport bearer can be used to transport DTCH/DCCH information received/to be sent on downlink Forward Access Channel (FACH) transport channels for multiple user equipment units (UEs).
The Iur interface has two planes, namely a control plane (CP) and a user plane (UP). In the control plane (CP), the serving radio network controller (SRNC) and drift radio network controller (DRNC) communicate using Radio Network Subsystem Application Protocol (RNSAP). Radio Network Subsystem Application Protocol (RNSAP) is transported over a reliable signaling bearer, as described in Third Generation Partnership Project (3GPP) Technical Specification 3G TS 25.423, v.3.4.0 “UTRAN Iur Interface RNSAP Signaling.”
The transport of data received/transmitted on the radio interface occurs in the user plane (UP). In the user plane (UP) the data is transported over unreliable transport bearers. For FACH transport channels, information related to multiple user equipment units (UEs) can be multiplexed onto one transport bearer, as described in Third Generation Partnership Project (3GPP) Technical Specification 3G TS 25.425, v.3.3.0 “UTRAN Iur Interface User Plane Protocols for CCH Data Streams.”
The serving radio network controller (SRNC) is responsible for establishing the necessary transport bearers between the serving radio network controller (SRNC) and the drift radio network controller (DRNC). For example, when Dedicated Control Channel (DCCH)/Dedicated Transport Channel (DTCH) communication with a new user equipment unit (UE) needs to be established over FACH and insufficient transport capability is available on any of the existing transport bearers, the serving radio network controller (SRNC) initiates establishment of a new transport bearer.
When a user equipment unit (UE) is using the RACH/FACH common channels (e.g., UE RRC state is CELL_FACH), the first message sent from the user equipment unit (UE) upon entering a new cell is a CELL-UPDATE RRC message. The CELL-UPDATE RRC message serves as a kind of registration message to let the UTRAN know the new position of the user equipment unit (UE). The UTRAN then decides how to handle this user equipment unit (UE). If UTRAN decides that the user equipment unit (UE) is to remain on common channels, the user equipment unit (UE) will be told to remain on common channels. The user equipment unit (UE) then monitors the FACH for any downlink transmission, and uses a certain RACH for uplink access. This case of the user equipment unit (UE) using common channels thus differs from a case of a user equipment unit (UE) using dedicated channels, since usage of dedicated channels may set up a soft handover situation in which new radio links or connection legs with the user equipment unit (UE) are established via the new cell.
When the user equipment unit (UE) is using the FACH transport channels, heretofore the serving radio network controller (SRNC) initiates a procedure known as the RNSAP Common Transport Channel Resource Initialization Procedure every time the user equipment unit (UE) moves from one cell to another cell in the DRNS, and the user equipment unit (UE) is to use common channels in the new cell. The Common Transport Channel Resource Initialization Procedure is a RNSAP procedure, initiated by the serving radio network controller (SRNC), which comprises messages transmitted between the serving radio network controller (SRNC) and the drift radio network controller (DRNC). Execution of the Common Transport Channel Resource Initialization Procedure is required to enable the drift radio network controller (DRNC) to perform certain activities, and to provide the serving radio network controller (SRNC) with, e.g., information concerning supported Transport Block (TB) sizes and flow control information. In addition, the Common Transport Channel Resource Initialization Procedure is executed when the serving radio network controller (SRNC) determines that another transport bearer over the Iur interface should be requested. Each of these are discussed briefly subsequently.
In every UTRAN cell, the data transported over the FACH transport channel needs to comply to certain length restrictions. These length restrictions may vary per cell and are used by the serving radio network controller (SRNC) for determining how to segment the Dedicated Control Channel (DCCH)/Dedicated Transport Channel (DTCH) data. When a user equipment unit (UE) moves from one cell to another, the serving radio network controller (SRNC) needs to be provided with information about the new applicable length restrictions. This information is provided by the CRNC in the RNSAP Common Transport Channel Resource Initialization Procedure.
Regarding flow control, the CRNC, in its role as drift radio network controller (DRNC), provides the serving radio network controller (SRNC) with a number of “credits”. These credits indicate how much data the serving radio network controller (SRNC) is allowed to send to the CRNC. If the SRNC has no credits remaining, it has to wait for new credits received from the CRNC before it can send further data to the CRNC. Thus, the credit mechanism provides a means to the CRNC to control the amount of data it will receive from a certain SRNC for a certain user equipment unit (UE). When the user equipment unit (UE) moves from one cell to another cell, the load conditions may vary. In such case, the currently specified solutions to the credit mechanism is always restarted. This means that the serving radio network controller (SRNC) has to obtain new credits from the CRNC before it can start transmission in the new cell. The credits applicable for the new cell are supplied by the CRNC to the serving radio network controller (SRNC) in the RNSAP Common Transport Channel Resource Initialization Procedure.
As indicated above, it is the serving radio network controller (SRNC) which decides if a new transport bearer should be established over the Iur interface or not. If the serving radio network controller (SRNC) decides that a new transport bearer shall be established, the RNSAP Common Transport Channel Resource Initialization Procedure provides the SRNC with relevant DRNC-related information to enable such establishment.
Execution of the Common Transport Channel Resource Initialization Procedure accounts for essentially half of the Iur signaling required to handle the situation in which a user equipment unit (UE) moves from one cell in the DRNS to another cell in the DRNS.
What is needed, therefore, and an object of the present invention, is a technique for decreasing Iur signaling when a cell change occurs in a DRNS.